Seismology is the study of earthquakes and the internal structure of the Earth and other planets, by both naturally and artificially generated seismic waves. Seismology seeks to both understand how seismic waves are propagate through the varied formations that compose the structure of the Earth, and to use waves to probe and understand the structure of the Earth. A seismic wave is an elastic wave generated by an impulse such as an earthquake or an explosion. Seismic waves may travel either along or near the Earth's surface (Rayleigh and Love waves) or through the Earth's interior (P and S waves).
Devices that detect and measure seismic waves are known as seismographs. The detection part is generally referred to as a seismometer and the recording part a seismogram.
There are a number of different types of seismic waves that prograte at different velocities through various geologic structures, and that reflect or refract varyingly at boundaries between the different underground features of our planet. Because of this, a seisomgraph at a particular location will record tremors in the earth that reveal a significant amount of information about Earth's subsurface geology. Using a network of seismographs at different locations relative to the source of the seismic waves, it is possible to develop a detailed understanding of the source of earthquakes and the many below ground geologic structures that interact with seismic waves. The use of ever more extensive networks of increasing sensitive seismographs with powerful computing systems is providing greater understanding to the scientists who study seismology (seismologists).
However, it is important to note that although seismology continues to make major strides in understanding earthquakes and the Earth's structure, much of the understanding developed is still rudimentary in many ways.
There are many practical applications that can use seismology for scientific investigation, including:
- understanding where and when earthquakes are more likely to occur (although the "when" part is still very rudimentary and can, currently, "predict" at time scales on hundreds of years or more);
- alerting those that might be impacted by a earthquake or a related phenomenon like a tsunami in time for the them to seek protection;
- understanding when volcanoes might erupt and alerting those impacted in time for the them to seek protection;
- exploring for natural resources like oil and natural gas; and
- monitoring the testing and use of nuclear weapons.
While there has always been intense concern and speculation about the causes of earthquakes, the serious theoretical and experimental scientific work did not get started until the nineteenth century, less than two hundred years ago.
On the theoretical front, a number of mathematicans began developing a basis for wave propagation in solid materials which could be applied to the seismic waves caused by earthquakes. These included:
- Siméon Denis Poisson (1781–1840);
- Augustin-Louis Cauchy (1789 –1857);
- George Gabriel Stokes (1819–1903); and,
- Lord Rayleigh (1842–1919)
The Irish scientist, Robert Mallet (1810–1881) both coined the term seismology and gave the field its first seminal publication when he present "The Dynamics of Earthquakes" to the Royal Irish Academy in 1846. Mallet recognized that earthquake waves radiate from a central point of origin which he termed the "epicenter", which could be located by observing the waves at several distance points and tracing them backward to their common source. This led him to recommend that networks of observatories should be established to monitor earthquakes. In 1851, Mallet began experimental work by detonating burried keys of gun powder and measuring when the seismic waves that they produced arrived at a distant location. This enabled him to make simple (although sometimes inaccurate) determinations of the speed of seismic waves through ground.
In 1875, Italian Filippo Cecchi (1822–1887) build what is considered to be the first true seismograph. It receives this recognition because it was the first device that was "expected to record the relative motion of a pendulum and the Earth as a function of time."
Cecchi's seismograph was quickly superceeded by the work of a group of British scientists working Japan. John Milne (1849–1913), Thomas Gray (1850–1908), James Alfred Ewing (1855-1935), were visiting professors at the Imperial College of Engineering in Tokyo when they began collaborating on seismographs in the late 1870s. Together, and independly, the three men made significant contribution to the development of seismographs and, over the following decades, to seismology.
As the nineteenth century came to a close seismology advanced as a field in many parts of the world as scientific studies and new observatories were undertaken in response to earthquake activity.
In 1897, German scientist Emil Wiechert (1861–1928) developed a seismometer with damping that overcame the earlier problem of oscillations caused by early tremors continuing and obscuring later tremors over the duration of an earthquake. On June 12 of that year, a major (magnitude ~8.1) earthquake struck Assam, India (now Myanmar) that was investigated British geologist Richard Oldham (1858–1936), who would become one of the most significant individuals in the field.
In 1899, Oldham proposed that there were seveal types of seismic waves, including compressional and distortional waves (later termed P- and S-waves) and surface waves. In 1906, Oldham's research led him to propose that the earth had a molten core.
1906 was a significant year for seismology other reasons too. On April 18, a major earthquake devastated the city of San Francisco and was recorded by the seismograph at the lick observatory. Harry Fielding Reid, (1859-1944), one of those undertook study in response to the earthquake proposed a new theory of "elastic rebound” which asserted that earthquakes were caused by the sudden movement of ground on either side of a geological fault. The movement was a "slip" triggered by stress and strain that had built up over a long period of time and were suddenly able to overcome the friction between the two sides of the fault.
Also in 1906, physicist Boris Borisovich Galitzen (1862–1916) developed the first electromagnetic seismograph and later established a network of seismic stations across Russia.
In 1909, following an earthquake near Zagreb, Croatia recoded by several seismographs, Andrija Mohorovi?i? (1857–1936) noted that seismic waves appeared to be reflected and refracted as though they were passing through a boundary and changing speed deep beneath the earth's surface and proposed that the earth had crust that overlay a deeper mantle.
A. E. H. Love (1863-1940) articulated a description for the behavior of surface seismic waves that are now known as Love Waves in 1911.
Emil Wiechert's research group included many important seismologists in the early part of the twentieth century. Karl Bernhard Zoeppritz (1881 –1908) and later Beno Gutenberg (1889–1960) published "travel-time tables" that provides data on how different types of seismic waves move through the earth. Gutenberg made a fairly accurately estimate of the depth of the earth’s fluid core - 2900 km.
By the 1920s, building on work of individuals like Ludger Mintrop (1880-1956), explosives-generated seismic waves were being used in the United States to explore for oil and other resources at relatively shallow depths.
In 1922, Herbert Hall Turner (1861–1930) presented evidence to show that earthquakes were not solely occurances near the Earth's surface, but could in fact, occur deep (300 and 700 km) beneath the Earth's surface. Kiyoo Wadati (1902 - 1995) extended Turner's insights with more accurate locations of deep earthquakes which revealed how parts of the earth's crusts are sliding under other parts (subduction). Hugo Benioff's (1899 – 1968) contribution to this area led to the term Wadati-Benioff zone to describe the part of a subducting plate where such earthquakes occur.
In 1935, Charles F. Richter of the California Institute of Technology developed the "local magnitude" or ML scale for moderate-size (3 < ML < 7) earthquakes in southern California which became more commonly known as the Richter scale.
A year later, Danish seismologist Inge Lehmann (1888–1993) presented case for more complex Earth's core with a solid inner core surrounded by a molten core.
Travel-time tables published in 1939 by H. Jeffries and K. Bullen achieved a level of accuracy that is recognized as useable in many purposes to the present.
Following World War II the advent of nuclear weapons led to a major investment in seismology in order to monitor the testing of nuclear devices in other parts of the world.
The ability to systematically observe earthquakes throughout the world revealed that they in fact primarily occur along specific belts which in the 1960's were revealed to be the boundaries between tectonic plates where crustal plates spread apart (e.g., mid-Atlantic Ridge), slide under other plates (subduction zones like in Japan and the Aleutians Islands), or slide past each other at transform boundaries (e.g., San Andreas fault).
Recent decades have seen a proliferation of sophisticated seismological research centers, observatories, and networks throughout the world. The Worldwide Standardized Seismograph Network (WWSSN - 1960), International Seismological Centre (ISC - 1964), Global Digital Seismograph Network (GDSN - 1978) and Incorporated Research Institutes for Seismology (IRIS - 1980) and four example mof major international efforts.
Seisomology has wide paractical application to resource discovery (see Seismic exploration).
In the realm of earthquake detection, early detection of seismic waves is built into warning sytems. Many lives were saved in Japan in 2011 because of alerts that were automatically sent throughout the country as soon a the earthquake began. Many individuals moved rapidly to higher ground and avoiding becoming additional victims to the tsunami that impacted the country.
- Aki, Keeiti and Paul G. Richards. 2009. Quantitative Seismology. 2nd Edition. University Science Books. ISBN-10: 9781891389634
- Shearer, Peter M. 2009. Introduction to Seismology. 2nd Edition. Cambridge University Press, Ca,bridge. ISBN-10: 0521708427
- Stein, Seth and Michael Wysession. 2002. An Introduction to Seismology. Wiley-Blackwell, New York. ISBN-10: 9780865420786